1. Field of the Invention
This invention relates to variable focal length lens systems consisting of a first lens unit possessing divergent refractive power and a movable second lens unit possessing convergent refractive power.
2. Description of the Prior Art
Variable focal length lens systems having a first divergent lens unit and a second convergent lens units are known. Such lens systems are disclosed in U.S. Pat. Nos. 4,099,844; 3,942,874 and 3,466,113. However, these lens system have an additional, third lens unit. These additional lens units increase the complexity and the cost of these lens systems. Each of these lens systems has six lens elements.
U.S. Pat. No. 4,585,314 discloses a verifocal lens system with a stationary first unit having divergent refractive power and a movable second unit possessing convergent refractive power. This lens system also comprises six lens elements. In addition this lens system is unfit for use as a taking lens (an objective lens) in a camera because it accepts a maximum full field angle of only 38 degrees.
U.S. Pat. No. 5,278,698 discloses magnifying projecting lens systems, some of which having a variable focal length. More specifically, the lens systems of embodiments 17-21 consist of a negative first lens unit (referred to as a negative first lens group), a positive second lens unit (referred to as a positive second lens group) and, a positive third lens unit (referred to as a positive third lens group). Although some of the embodiments have as few as four lens elements (see embodiment 19), all three of the lens units move in order to provide a variable focal length system. The patent states (col. 9, lns. 67,68 and col. 10, ln. 1) that "The focal length is changed by changing a distance between the second lens group and the third lens group and by moving the whole lens! system." Finally, these lens systems accept a field angle of only 50 degrees, have an F-number of F/4.8 to F/5.9, and have a zoom ratio of only 1.5.
Further need exists for a simple variable focal length (verifocal) lens system that has only one zooming lens unit, accepts a field angle of at least 60 degrees, provides larger numerical apertures and is capable of zoom ratios over 1.5, and that also has a small number of lens elements.